There are many circumstances wherein it is necessary or desirable to determine the geographic location of an emitter of electromagnetic radiation, such as a radar system, a communications facility or device or an emergency beacon or transmitter. Typical applications may include, for example, military signal intelligence (SIGINT) and electronic intelligence (ELINT) operations for locating radar or communications facilities, and air, land and sea rescue operations wherein it is necessary to locate an emergency beacon or transmitter, such as used in aircraft and vessels, or communications devices ranging from conventional or emergency radio devices to cell phones.
Such applications and operations are characterized by common requirements that are, in turn, imposed by general, common characteristics of the target emitters to be located and the situations or circumstances under which the target emitters are to be located. For example, the signal transmitted by a target emitter may be of relatively low power, as in the case of emergency beacons or emergency radios, or may be masked, distorted or effectively reduced by terrain or weather conditions, and such conditions may be intentionally imposed in, for example, military or otherwise hostile situations. In addition, the time available or permissible for locating a target emitter may be limited in both military and civil situations, that is, and for example, in military counter-measures operations or in search and rescue operations, and the resources available for target emitter location may be limited.
As such, it is generally necessary or desirable for a system for locating target emitters to be mobile, that is, to be readily transportable into the general geographical location of a target emitter on an aircraft, vehicle or vessel, both to bring the locator system into range of the target emitter and to allow the locator system to search as large an area as possible in the minimum time. It is also desirable that a locator system be transported and employed in and from a single platform, whether an aircraft, vessel or vehicle, as the use of a single platform reduces the system cost, reduces demand on frequently limited resources and allows a greater area or number of areas to be searched when multiple platforms are available. A single platform system also eliminates the complexity and time delays inherent in deploying and coordinating multiple cooperatively operating platforms.
The locator system must be capable of identifying the geographic location of a target emitter with the greatest possible accuracy as insufficient accuracy in locating a target emitter may render counter-measures ineffective in military situations and may unacceptably delay locating or reaching the target emitter in civil situations, such as search and rescue operations, particularly in difficult terrain or weather conditions. In addition, the locator system should be capable of locating as wide a range of target emitter types as possible, and correspondingly over as wide a range of the electromagnetic spectrum as possible, to allow a given locator system to be employed in as wide a range of applications and situations as possible.
There are a number of factors that determine and limit the characteristics and capabilities of an emitter location system, and in particular a single platform, mobile emitter location system, are numerous and inter-related. For example, current methods for single platform emitter location are based upon determining multiple direction finding (DF) bearings, often referred to as DF “cuts”, to the target emitter at points along a path traversed by the locator platform, such as the flight path of an aircraft. Each “cut” is an attempt to determine the direction of the emitter relative to the locator platform at the point the “cut” is taken by using an amplitude or phase detecting directional antenna and receiver array to determine the direction of the strongest signal component or the phase gradient, that is, the direction of propagation, of the wavefront of the received signal. Successive DF cuts may be used to determine a Line of Bearing (LOB) “fan” of DF cuts, with the location of the target emitter being taken as the point of intersection of the DF bearings forming the LOB fan.
These method of the prior art are, however, subject to significant limitations and problems. For example, signal propagation factors between the emitter and the locator system path at various points, such as variations in propagation conditions, local multipath distortions, multiple propagation paths and reflections, will result in significant errors in the measured gradients of the wavefront and this significant errors in the measured bearings between the locator system and the target emitter.
One of if not the most significant problem in the direction finding methods of the prior art is that of multipathing, that is, the tendency for a received signal to appear to arrive from multiple sources separated from the true source. This phenomenon is well known to communications engineers, as evidenced, for example, by R. H. Clarke: “A Statistical Theory for Mobile Radio Communications,” Bell System Technical Journal, July 1968, 47, pp. 957-1000).
In brief, multipath sources typically appear to surround the receiving unit and to have effectively random radiation patterns and arise from the reflection or refraction of the transmitted signal by “scatterers”, which may be any element of the environment capable of reflecting the original signal or of refracting the original signal around themselves.
Conventional direction finding systems typically employ two or more receiving antennas spaced apart from one another along a “baseline” and compare the amplitudes or phases of the signals received at the antennas to determine the direction to the transmitter. This method is, however, historically subject to systemic errors for a number of reasons. For example, if the antennas are spaced too closely there will be correlation between the multipath components of the received signal, and between the multipath components and the direct arrival component, resulting in an induced multipath bias error that cannot be “washed out” even by time integration of the received signal components. If, however, the antennas are spaced too far apart, such as more than one wavelength apart, the multipath and direct arrival components will be decorrelated, but there will be phase ambiguity in the received signals because the received direct arrival component, for example, will contain more than one wavelength.